Operations in oil and gas boreholes typically involve the use of pipes formed from sections joined end-to-end to form long tubular sections. One example of such an operation is conventional rotary drilling. In this operation, a drill string, formed from sections of pipe is used to rotate a drill bit to drill through underground formations. During the drilling process, drilling fluid is pumped down the inside of the drill string to exit through the drill bit and return to the surface carrying drilled cuttings. The drilling fluid also acts to support the borehole mechanically and balance the pressure of formation fluids and so the borehole is kept full of fluid during the drilling operation. The drill string is formed from sections of pipe (typically 10 m long and often called ‘joints’). As drilling progresses, joints can be added to increase the length of the drill string in the borehole. Likewise, joints are removed from the drill string if it is necessary to remove it from the borehole for any reason (replacement of bit, logging operations, completion, etc.).
Problems can arise when a cable is required to be run through the pipe, perhaps to provide a telemetry link between the bottom hole apparatus and the surface.
One method to provide a cable in a pipe is to first assemble the pipe from the joints so that it is at the required depth, and then to feed a cable down through the pipe from the surface.
When the pipe is ready to be withdrawn from the hole, the cable is then fully retracted to the surface and the disassembly of the joints in the normal way can commence.
This method cannot provide a link while the pipe is being run into or pulled out of the well, and if it is required that the pipe be increased or decreased in length by a small amount, then the cable must be disconnected and fully retracted before any joints can be added or removed.
Threading the cable through all of the separate the joints first and then building up the pipe by using the next joint that is available on the line is not generally practical in most well operations.
This invention provides a method and apparatus for hanging a cable inside a pipe while it is able to be run in or out of the subterranean formation and which overcomes the difficulties of existing methods as detailed above. The invention is based on the use of a float that floats at the surface of the fluid filling the borehole.